Reducing viewport slowness while working on a .blend with lots of particles?
Expanding on stacker's answer, in case you are not using child particles, you can also reduce the percentage of real particles displayed in the viewport in Particle settings > Display:



Note that you should bake your simulation with this set to 100%, and any simulations with it at less than 100% will not be the same a simulations with it at 100%.

You can also set the particles to draw in the viewport as something other than what they will appear as in the render.



This will probably increase viewport performance a lot